The present invention relates to an intrusion detection system useful in perimeter protection. More particularly, the invention relates to an intrusion detection system using at least two fluid-filled flexible tube members containing a fluid capable of transmission of energy pulses in response to pressure applied to the external portion of the flexible tube members. The flexible tube members are placed at a certain distance below ground level along the boundary to be protected. Energy pulses are detected by transducers located at least at the ends of the flexible tube members which are capable of converting mechanical energy (vibration) pulses into electrical signals. The novel intrusion detection system further comprises control and indicating equipment including an evaluating circuit to produce an alarm signal if pressure changes or vibrations in the fluid in the tubes indicate the penetration of the boundary to be protected.
U.S. Pat. No. 4,400,695 discloses an intrusion detection system for perimeter surveillance, i.e., for the surveillance of the boundaries of outdoor areas against unauthorized trespass by intruders or against breach of the boundaries by vehicles. This system utilizes two fluid-filled tubes buried within a few feet of each other. Seismic disturbances (vibrations) caused by an intruder are transformed into pressure pulses and transmitted to transducers positioned at the ends of the tubes. The output signals of these transducers are evaluated in an evaluating circuit. In order to minimize false alarms, an alarm signal is only produced if the difference between the signals of the two tubes exceeds a predetermined threshold value. The location of the intrusion can be detected by measuring the time difference between receipt of the mechanical impulse at the transducers at both ends of the tubes. Accordingly, the intrusion detection system only evaluates a single physical quantity for producing an alarm signal, i.e., the amplitude of the seismic disturbances or vibrations. Such a system is not suitable to distinguish between an intruder, i.e., a genuine alarm situation and other causes of vibrations, for instance vibrations caused by small animals crossing the boundary, vibrations caused by distant vehicles or vibrations caused by weather or other environmental conditions.
As a result of this limitation, false alarms frequently occur. Furthermore, it is difficult, if not impossible, to detect the location of the intrusion occurrence if several pulses are produced simultaneously or within short time intervals, for instance by several intruders or by a single intruder producing signals in rapid succession, e.g., footsteps.
It is an object of the present invention to overcome the above-described disadvantages of the prior art intrusion detection systems and in particular to provide an intrusion detection system which detects an intrusion occurrence with improved selectivity and sensitivity so as to distinguish between vibrations resulting from environmental conditions and genuine intrusive acts. A further object is to provide an intrusion detection system with improved security against sabotage and circumvention and which permits precise identification of the location of an intrusion occurrence.